The story depicts an extraterrestrial invasion that begins in the small fictional California town of Santa Mira. Alien plant spores have fallen from space and grown into large seed pods, each one capable of reproducing a duplicate replacement copy of each human. As each pod reaches full development, it assimilates the physical characteristics, memories, and personalities of each sleeping person placed near it; these duplicates, however, are devoid of all human emotion. Little by little, a local doctor uncovers this "quiet" invasion and attempts to stop it.

The slang expression "pod people" that arose in late 20th Century American culture references the emotionless duplicates seen in the film.[2]

In 1994 Invasion of the Body Snatchers was selected for preservation in the United States National Film Registry by the Library of Congress for being "culturally, historically, or aesthetically significant."

Psychiatrist Dr. Hill is called to the emergency room of a hospital, where a screaming man is held in custody. Dr. Hill agrees to listen to his story. The man identifies himself as Dr. Miles Bennell, and he recounts, in flashback, the events leading up to his arrest and arrival at the hospital:

In Santa Mira, California, Miles Bennell, a local doctor, sees a number of patients who believe their loved ones have been replaced by impostors. Returning from a trip, Miles meets his former girlfriend, Becky Driscoll, who has herself recently come back to town after a recent divorce. Becky tells him that her cousin Wilma has the same fear about her Uncle Ira, with whom she lives. Dr. Dan Kauffman, a psychiatrist in the town, assures Bennell that these cases are merely an "epidemic of mass hysteria."

That same evening, Bennell's friend Jack Belicec finds a body with his physical features, though it's not fully developed; later, another body is found in the cellar of Becky's home that is a copy of her. When Bennell calls Kauffman to the scene, the bodies have mysteriously disappeared, and Kauffman informs Bennell that he is falling for the same hysteria. The following night, Bennell, Becky, Jack, and Jack's wife Teddy again find duplicates of themselves, emerging from large seed pods in Dr. Bennell's greenhouse. They conclude that the townspeople are being replaced while asleep by exact physical copies. Miles tries to make a long distance call for help from federal authorities, but the phone operator claims that all long-distance lines are busy; Jack and Teddy drive off to seek help in the next town. Bennell and Becky discover that by now all of the town's inhabitants have been replaced and are devoid of humanity; they flee to Bennell's office to hide for the night.

The next morning, they see truckloads of the large pods heading to neighboring towns, to be planted by "pod people" and used to replace surrounding populations little by little. Kauffman and Jack, both of whom are "pod people" by now, arrive at Bennell's office and reveal that an extraterrestrial life form is responsible for the invasion. After their takeover, they explain, life loses its frustrating complexity, because all emotions and sense of individuality vanish. Bennell and Becky escape and hide in an abandoned mine outside of town. Bennell comes upon a nearby farm and discovers more large seed pods being grown by the hundreds. While he is gone, Becky falls asleep and is transformed; when he returns and kisses her he realizes what has happened. She calls out to the pursuing "pod people". Bennell, now panicking, runs and eventually finds himself on a crowded highway. After seeing a truck filled with pods, he frantically screams to passing motorists, "They're here already! You're next! You're next!"

As Bennell finishes his story at the hospital, Dr. Hill and the on-duty doctor doubt his account until an injured truck driver, involved in a highway accident, is brought into the emergency room. He was found in his wrecked truck buried under a load of giant seed pods. Both doctors realize that Bennell's story is true, and they immediately call the federal authorities.

Jack Finney's novel ends with the extraterrestrials finally leaving Earth after they find that humans are offering strong resistance, despite having little reasonable chance against the alien invasion; the "pod people" have a life span of no more than five years, so five years after taking over the last human being, the invaders would then have to seek out a new world with new life forms as hosts, leaving behind a depopulated Earth.[2]

Invasion of the Body Snatchers was originally scheduled for a 24-day shoot and a budget of US$454,864. The studio later asked Wanger to cut the budget significantly. The producer proposed a shooting schedule of 20 days and a budget of $350,000.[3]

Originally, producer Wanger and Siegel wanted to film Invasion of the Body Snatchers on location in Mill Valley, California,the town just north of San Francisco, that Jack Finney described in his novel.[3] In the first week of January 1955, Siegel, Wanger and screenwriter Daniel Mainwaring visited Finney to talk about the film version and to look at Mill Valley. The location proved too expensive and Siegel with Allied Artist executives found locations resembling Mill Valley in Sierra Madre, Chatsworth, Glendale, the Los Feliz neighborhood, Bronson and Beachwood Canyons, all of which would make up the town of "Santa Mira" for the film.[3] In addition to these outdoor locations, much of the film was shot in the Allied Artists studio on the east side of Hollywood.[2]

Invasion of the Body Snatchers was shot by cinematographer Ellsworth Fredericks in 23 days between March 23 and April 18, 1955. The cast and crew worked a six-day week with Sundays off.[3] The production went over schedule by three days because of night-for-night shooting that Siegel wanted. Additional photography took place in September 1955, filming a frame story on which the studio had insisted (see Original intended ending). The final budget was $382,190.[2]

The project was originally named The Body Snatchers after the Finney serial.[6] However, Wanger wanted to avoid confusion with the 1945 Val Lewton film The Body Snatcher. The producer was unable to come up with a title and accepted the studio's choice, They Come from Another World and that was assigned in summer 1955. Siegel objected to this title and suggested two alternatives, Better Off Dead and Sleep No More, while Wanger offered Evil in the Night and World in Danger. None of these were chosen, and the studio settled on Invasion of the Body Snatchers in late 1955.[6] The film was released at the time in France under the mistranslated title "L'invasion des profanateurs de sépultures" (literally: Invasion of the defilers of tombs), which remains unchanged today.

Wanger wanted to add a variety of speeches and prefaces.[7] He suggested a voice-over introduction for Miles.[8] While the film was being shot, Wanger tried to get permission in England to use a Winston Churchill quotation as a preface to the film. The producer sought out Orson Welles to voice the preface and a trailer for the film. He wrote speeches for Welles' opening on June 15, 1955, and worked to persuade Welles to do it, but was unsuccessful. Wanger considered science fiction author Ray Bradbury instead, but this did not happen, either.[8] Mainwaring eventually wrote the voice-over narration himself.[6]

The studio scheduled three film previews on the last days of June and the first day of July 1955.[8] According to Wanger's memos at the time, the previews were successful. Later reports by Mainwaring and Siegel, however, contradict this, claiming that audiences could not follow the film and laughed in the wrong places. In response the studio removed much of the film's humor, "humanity" and "quality," according to Wanger.[8] He scheduled another preview in mid-August that also did not go well. In later interviews Siegel pointed out that it was studio policy not to mix humor with horror.[8]

Wanger saw the final cut in December 1955 and protested the use of the Superscopeaspect ratio.[6] Its use had been included in early plans for the film, but the first print was not made until December. Wanger felt that the film lost sharpness and detail. Siegel originally shot Invasion of the Body Snatchers in the 1.85:1 aspect ratio. Superscope was a post-production laboratory process designed to create an anamorphic print from non-anamorphic source material that would be projected at an aspect ratio of 2.00:1.[6][9]

Both Siegel and Mainwaring were satisfied with the film as shot. It was originally meant to end with Miles screaming as truckloads of pods pass him by.[7] The studio, wary of a pessimistic conclusion, insisted on adding a prologue and epilogue to the movie suggesting a more optimistic outcome to the story, which is thus told mainly in flashback. In this version the movie begins with a ranting Bennell in custody in a hospital emergency ward. He then tells an arriving doctor (Whit Bissell) his story. In the closing scene pods are found at a highway accident, confirming his warning. The Federal Bureau of Investigation is notified, possibly in time to save the Earth.[2]

Mainwaring scripted this framing story and Siegel shot it on September 16, 1955, at the Allied Artists studio.[6] In a later interview Siegel complained, "The film was nearly ruined by those in charge at Allied Artists who added a preface and ending that I don't like."[10] In his autobiography Siegel added that "Wanger was very much against this, as was I. However, he begged me to shoot it to protect the film, and I reluctantly consented […]".[11]

Though disapproved of by most reviewers, George Turner (in American Cinematographer)[14] and Danny Peary (in Cult Movies)[15] endorsed the subsequently added frame story. Nonetheless, Peary emphasized that the added scenes changed significantly what he saw as the film's original intention (see Themes).

When the film was released domestically in February 1956, many theaters displayed several pods made of papier-mâché in theater lobbies and entrances, along with large lifelike black and white cutouts of McCarthy and Wynter running away from a crowd. The film made more than $1 million in the first month, and in 1956 alone made more than $2.5 million in the U.S.[2] When the British release (with cuts imposed by the British censors[16]) took place in late 1956, the film earned more than a half million dollars in ticket sales.[6]

Some reviewers saw in the story a commentary on the dangers facing America for turning a blind eye to McCarthyism,"Leonard Maltin speaks of a McCarthy-era subtext."[17] or of bland conformity in postwar Eisenhower-era America. Others viewed it as an allegory for the loss of personal autonomy in the Soviet Union or communist systems in general.[18] For the BBC, David Wood summarized the circulating popular interpretations of the film as follows: "The sense of post-war, anti-communist paranoia is acute, as is the temptation to view the film as a metaphor for the tyranny of the McCarthy era."[19] Danny Peary in Cult Movies pointed out that the addition of the framing story had changed the film's stance from anti-McCarthyite to anti-communist.[15] Michael Dodd of The Missing Slate has called the movie "one of the most multifaceted horror films ever made", arguing that by "simultaneously exploiting the contemporary fear of infiltration by undesirable elements as well as a burgeoning concern over homeland totalitarianism in the wake of Senator Joseph McCarthy’s notorious communist witch hunt, it may be the clearest window into the American psyche that horror cinema has ever provided".[20]

In An Illustrated History of the Horror Film, Carlos Clarens saw a trend manifesting itself in Science Fiction films, dealing with dehumanization and fear of the loss of individual identity, being historically connected to the end of "the Korean War and the well publicized reports of brainwashing techniques".[21] Comparing Invasion of the Body Snatchers with Robert Aldrich'sKiss Me Deadly and Orson Welles' Touch of Evil, Brian Neve found a sense of disillusionment rather than straightforward messages, with all three films being "less radical in any positive sense than reflective of the decline of [the screenwriters'] great liberal hopes".[22]

Despite a general agreement among film critics regarding these political connotations of the film, actor Kevin McCarthy said in an interview included on the 1998 DVD release that he felt no political allegory was intended. The interviewer stated that he had spoken with the author of the novel, Jack Finney, who professed no specific political allegory in the work. DVD commentary track, quoted in Feo Amante's homepage.[23]

In his autobiography, I Thought We Were Making Movies, Not History, Walter Mirisch writes: "People began to read meanings into pictures that were never intended. The Invasion of the Body Snatchers is an example of that. I remember reading a magazine article arguing that the picture was intended as an allegory about the communist infiltration of America. From personal knowledge, neither Walter Wanger nor Don Siegel, who directed it, nor Dan Mainwaring, who wrote the script nor original author Jack Finney, nor myself saw it as anything other than a thriller, pure and simple."[24]

Don Siegel spoke more openly of an existing allegorical subtext, but denied a strictly political point of view: "[…] I felt that this was a very important story. I think that the world is populated by pods and I wanted to show them. I think so many people have no feeling about cultural things, no feeling of pain, of sorrow. […] The political reference to Senator McCarthy and totalitarianism was inescapable but I tried not to emphasize it because I feel that motion pictures are primarily to entertain and I did not want to preach."[25] Film scholar J. P. Telotte wrote that Siegel intended for pods to be seductive; their spokesperson, a psychiatrist, was chosen to provide an authoritative voice that would appeal to the desire to "abdicate from human responsibility in an increasingly complex and confusing modern world."[26]

Largely ignored by critics on its initial run,[14]Invasion of the Body Snatchers received wide critical acclaim in retrospect and is considered one of the best films of 1956.[27][28][29] The film holds a 98% "Fresh" rating on the review aggregate Web site Rotten Tomatoes.[30]

In recent years, critics such as Dan Druker, Chicago Reader have hailed the film as a "genuine Sci-Fi classic".[31] (Leonard Maltin) described Invasion of the Body Snatchers as "influential, and still very scary".[17]Time Out called the film, one of the "most resonant" and "one of the simplest" of the genre.[32]

Invasion of the Body Snatchers was selected in 1993 for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".[33] In June 2008, the American Film Institute revealed its "Ten top Ten" — the best ten films in ten "classic" American film genres — after polling over 1,500 people from the creative community. Invasion of the Body Snatchers was acknowledged as the ninth best film in the science fiction genre.[34] The film was also placed on AFI's AFI's 100 Years ... 100 Thrills, a list of America's most heart-pounding films.[35] The film was included on Bravo's 100 Scariest Movie Moments.[36] Similarly, the Chicago Film Critics Association named it the 29th scariest film ever made.[37]Time magazine included Invasion of the Body Snatchers on their list of 100 all-time best films,[38] the top 10 1950s Sci-Fi Movies,[39] and Top 25 Horror Films.[40]

The film was released on DVD in 1998 by U.S.-label Republic (an identical re-release by Artisan followed in 2002); it includes the Superscope version plus a 1.375:1 Academy ratio version. The latter is not the original full frame edition, but a pan and scan reworking of the Superscope edition that loses visual detail.